Adeno-associated virus (AAV) may be considered as one of the most promising viral vectors for human gene therapy. AAV has the ability to efficiently infect dividing as well as non-dividing human cells, the AAV viral genome integrates into a single chromosomal site in the host cell's genome, and most importantly, even though AAV is present in many humans it has never been associated with any disease. In view of these advantages, recombinant adeno-associated virus (rAAV) is being evaluated in gene therapy clinical trials for hemophilia B, malignant melanoma, cystic fibrosis, and other diseases.
Host cells that sustain AAV replication in vitro are all derived from mammalian cell types. Therefore, rAAV for use in gene therapy has thus far mainly been produced on mammalian cell lines such as e.g. 293 cells, COS cells, HeLa cells. KB cells, and other mammalian cell lines (see e.g. U.S. Pat. No. 6,156,303, U.S. Pat. No. 5,387,484, U.S. Pat. No. 5,741,683, U.S. Pat. No. 5,691,176, U.S. Pat. No. 5,688,676, US 20020081721, WO 00/47757, WO 00/24916, and WO 96/17947). rAAV vectors are typically produced in such mammalian cell culture systems by providing DNA plasmids that contain the therapeutic gene flanked by the origin of AAV replication (inverted terminal repeats or ITRs), genes for AAV replication proteins Rep78, Rep68, Rep52, and Rep40, and genes for virion or structural proteins VP1, VP2, and VP3. In addition, a plasmid containing early genes from adenovirus (E2A, E4ORF6, VARNA) is provided to enhance the expression of the AAV genes and improve vector yield (see e.g. Grimm et al., 1998, Hum. Gene Ther. 2: 2745-2760). However, in most of these mammalian cell culture systems, the number of AAV particles generated per cell is on the order of 104 particles (reviewed in Clark, 2002, Kidney Int. 61(Suppl. 1): 9-15). For a clinical study, more than 1015 particles of rAAV may be required. To produce this number of rAAV particles, transfection and culture with approximately 1011 cultured human 293 cells, the equivalent of 5,000 175-cm2 flasks of cells, would be required, which means transfecting up to 1011 293 cells. Therefore, large scale production of rAAV using mammalian cell culture systems to obtain material for clinical trials has already proven to be problematic, production at commercial scale may not even be feasible. Furthermore there is always the risk, that a vector for clinical use that is produced in a mammalian cell culture will be contaminated with undesirable, perhaps pathogenic, material present in the mammalian host cell.
To overcome these problems of mammalian productions systems, recently, an AAV production system has been developed using insect cells (Urabe et al., 2002, Hum. Gene Ther. 13: 1935-1943; US 20030148506 and US 20040197895). For production of AAV in insect cells some modifications were necessary in order to achieve the correct stoichiometry of the three AAV capsid proteins (VP1, VP2 and VP3), which relies on a combination of alternate usage of two splice acceptor sites and the suboptimal utilization of an ACG initiation codon for VP2 that is not accurately reproduced by insect cells. To mimic the correct stoichiometry of the capsid proteins in insect cells Urabe et al. (2002, supra) use a construct that is transcribed into a single polycistronic messenger that is able to express all three VP proteins without requiring splicing and wherein the most upstream initiator codon is replaced by the suboptimal initiator codon ACG. In co-pending application (PCT/NL2005/050018) the present inventors have further improved the infectivity of baculovirus-produced rAAV vectors based production by further optimisation of the stoichiometry of AAV capsid proteins in insect cells.
For expression of the AAV Rep proteins in the AAV insect cell expression system as initially developed by Urabe et al. (2002, supra), a recombinant baculovirus construct is used that harbours two independent Rep expression units (one for Rep78 and one for Rep52), each under the control of a separate insect cell promoter, the ΔIE1 and PolH promoters, respectively. In this system, the ΔIE1 promoter, a much weaker promoter than the PolH promoter, was chosen for driving Rep78 expression since it is known that in mammalian cells a less abundant expression of Rep78 as compared to Rep52 favours high vector yields (Li et al., 1997, J. Virol. 71: 5236-43; Grimm et al., 1998, supra).
More recently however, Kohlbrenner et al. (2005, Mol. Ther. 12: 1217-25) reported that the baculovirus construct for expression of the two Rep protein, as used by Urabe et al., suffers from an inherent instability. By splitting the palindromic orientation of the two Rep genes in Urabe's original vector and designing two separate baculovirus vectors for expressing Rep52 and Rep78, Kohlbrenner et al. (2005, supra) increased the passaging stability of the vector. However, despite the consistent expression of Rep78 and Rep52 from the two independent baculovirus-Rep constructs in insect cells over at least 5 passages, rAAV vector yield is 5 to 10-fold lower as compared to the original baculovirus-Rep construct designed by Urabe et al. (2002, supra).
There is thus still a need to overcome the above serious limitations of large scale (commercial) production of AAV vectors in insect cells. Thus it is an object of the present invention to provide for means and methods that allow for stable and high yield (large scale) production of AAV vectors in insect cells.